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냉매를 이용한 나노선 표면에서의 강제대류 비등열전달 특성 평가
최지홍(Geehong Choi),이동휘(Donghwi Lee),심동일(Dong-Il Shim),김범석(Beom Seok Kim),신상우(Sangwoo Shin),조형희(Hyung Hee Cho) 대한기계학회 2014 대한기계학회 춘추학술대회 Vol.2014 No.11
Flow boiling is combined heat transfer mode of convective flow and phase change, and has been applied to electronic cooling and refrigeration system. In the early stage of the study, flow characteristics including the mass flow rate, fluid temperature, fluid pressure, and flow structure were controlled to enhance heat transfer. As the micro-electromechanical system (MEMS) technology developed, various micro/nano-materials have been applied to flow boiling. However, it is hard to analyze heat transfer mechanism on nano-structured surface because a number of hydrodynamics and heat transfer modes are mixed in flow boiling. In this study, silicon nanowires were coated on heat transfer surface, and refrigerant FC-72 was used as a working fluid. FC-72 is highly wetting fluid, which has small contact angle around 10°, and interesting wetting phenomena was observed using FC-72. Heat transfer characteristics of flow boiling were presented nonmonotonically according to the height of the silicon nanowires and the Reynolds number of convective flow. We explain the nonmonotonic distribution of critical heat flux using the morphology of nano-structure affecting to the fluid supply near the heat transfer surface.
최지홍(Geehong Choi),심동일(Dong-Il Shim),이남규(Namkyu Lee),이동휘(Donghwi Lee),조형희(Hyung Hee Cho) 대한기계학회 2015 대한기계학회 춘추학술대회 Vol.2015 No.11
Flow boiling heat transfer is combined boiling heat transfer with forced convection. Thus flow boiling characteristics were changed according to the flow condition and surface characteristics, which were surface roughness and wettability. In addition, the effect of surface characteristics was changed as the forced convection intensified. In this study, the effect of surface roughness was evaluated according to the mass flow rate in horizontal rectangular channel. The mass flow rate was maintained to Re = 10,000 and 18,000 to control forced convection condition, and the surface characteristics were changed by silicon nanowires coating with various heights. Dielectric fluid (FC-72) was used as working fluid, and the experimental condition was controlled to saturated temperature and atmosphere pressure. Heat transfer coefficient was declined on the rise again as the nanowires’ height increased for all mass transfer rate condition, and we analyzed the results with surface morphology of Si nanowires.
강제대류 비등열전달에서의 표면거칠기가 임계열유속에 미치는 영향
최지홍(Geehong Choi),이남규(Namkyu Lee),김범석(Beom Seok Kim),김경민(Kyung Min Kim),조형희(Hyung Hee Cho) 대한기계학회 2016 대한기계학회 춘추학술대회 Vol.2016 No.12
Flow boiling is one of powerful heat transfer mode, which is combined convection and phase change. The maximum cooling capacity of flow boiling is evaluated by critical heat flux (CHF). The performance of CHF is affected by not only fluid condition but also surface characteristics, which are surface roughness and wettability. The various micro/nano structures are increased surface roughness wettability, and many studies have been reported that CHF was improved by surface treatments using micr/nano structures. However, the mechanism of how each surface characteristic enhances CHF is not clearly understanding because surface roughness and wettability are related each other. In this study, we tried to evaluate the independent effect of surface characteristics on CHF in flow boiling using silicon nanowires. Secondary wettability, evaluated by wicking rate, was diminished by decreased surface tension of working fluid. Experiments were conducted in saturated condition of working fluid (DI water), and the mass flow rate was maintained in Re = 12,000.